The Role of Musculoskeletal Models in Patient Assessment and Treatment

The management of gait abnormalities in persons with cerebral palsy is a challenging issue. Theoretically, gait abnormalities can be diminished by first identifying the biomechanical factors that contribute to abnormal movement and then either decreasing the muscle forces that disrupt normal movement (e.g. via muscle-tendon lengthening or botulinum toxin injections), and/or increasing the muscle and ground reaction forces that have the potential to improve movement (e.g. via strengthening exercises, orthoses, or derotational osteotomies). However, different patients with cerebral palsy exhibit varying degrees of neurologic impairment, spasticity, weakness, muscle contracture and bone deformity, suggesting that gait deviations arise from a variety of sources, each requiring a different treatment. Treatment planning is further complicated because there is currently no scientific basis for determining how patients’ neuro-musculoskeletal impairments contribute to abnormal movement. The static muscle tests performed during a patient’s physical exam (Chapter 5) and the kinematic, kinetic, and electromyographic (EMG) data obtained from gait analysis (Chapters 6–9) are not always sufficient to identify the biomechanical source of a patient’s abnormal gait or to predict the consequences of treatments. This limitation exists, in part, because the transformation from EMG patterns to motion is extremely complex (Fig. 11.1) and because the effects of common surgical procedures on muscle-tendon mechanics and musculoskeletal geometry are not easily measured. This chapter describes how computer simulations of the musculoskeletal system can be used, in combination with gait analysis, to enhance our understanding of movement abnormalities and to provide a theoretical basis for planning treatments. Imagine the following hypothetical scenario. A child with a troublesome gait abnormality visits a cerebral palsy clinic. The child undergoes a routine physical exam, a gait analysis, and perhaps a medical imaging study. A computer model of the child’s musculoskeletal system is created that characterizes the force-generating capacity of the muscles, the geometric relationships between the muscles, tendons and bones, the kinematics of the joints, and the inertial properties of the body (see the shaded region of Fig. 11.1). The model is driven by a set of muscle excitation signals, and the resulting motion of the model is governed by mathematical equations that describe the activation dynamics of muscle, the contraction dynamics of muscle, and the multi-joint dynamics of the body during walking. The muscle excitation signals are specified such that the computer model “walks”